Cell re-selection utilizing system information length

ABSTRACT

A method for performing cell re-selection in a cellular network includes a subscriber terminal measuring received powers of neighbour cells in accordance with system information received from a current cell, selecting one of the neighbour cells as a new cell, the subscriber terminal receiving a part of the system information sent by the new cell, calculating the time used for receiving the system information of the new cell by employing the length information in the system information part sent by the new cell, and deciding whether to continue the re-selection of said new cell on the basis of the calculated time.

This application is a divisional of U.S. application Ser. No.09/419,171, filed 15 Oct. 1999.

FIELD OF INVENTION

The invention relates to implementing cell re-selection in a cellularnetwork. The invention particularly relates to a cellular network usingGPRS (General Packet Radio Service), and to a PBCCH (Packet BroadcastControl Channel) used therein, and to mapping system information ontothe PBCCH.

BACKGROUND OF THE INVENTION

When a subscriber terminal moves, measures must be taken which make surethat the subscriber terminal always listens to the base station that isbest heard. The subscriber terminal receives system information sent bythe base station on a control channel informing which neighbour basestations the subscriber terminal should also listen to. When thesubscriber terminal detects that the received power of a neighbour cellsignal it has listened to and possibly some other parameters are betterthan that of the cell, whose control channel the subscriber terminal hasbeen listening to, then the subscriber terminal decides to perfrom cellre-selection. A network part of a cellular network, i.e. the networkinfrastructure including, for example, base stations, base stationcontrollers and mobile services switching centres, can also discover theneed for cell re-selection and inform the subscriber terminal about it.In connection with cell re-selection the subscriber terminal has toreceive the system information of a new cell sent on its controlchannel.

In an ordinary GSM system the system information has a standardstructure. In a cellular network using GPRS the structure and length ofthe system information sent by different cells may vary a lot. Thesubscriber terminal does not know in advance how long it takes to readthe system information. In demanding packet transmission applicationsthis may result in a situation, where a fairly long break may occur inperforming packet transmission so that the user detects the break as adelay in the application operation. The user may interpret the delay aspoor quality service.

BRIEF DESCRIPTION OF THE INVENTION

It is thus an object of the invention to provide a method and anapparatus implementing the method so as to solve the above problems.This is achieved with the method presented below. The method performscell re-selection in a cellular network comprising a subscriber terminalmeasuring received powers of neighbour cells in accordance with systeminformation received from a current cell; one of the neighbour cells asa new cell; the subscriber terminal receiving a part of the systeminformation sent by the new cell. In this method the time used forreceiving the system information of the new cell is calculated byemploying the length information in the system information part sent bythe new cell.

The invention also relates to a subscriber terminal comprising a radioconnection to a current cell base station of a cellular network; meansfor measuring received powers of neighbour cells in accordance withsystem information received from a current cell; means for discoveringthe need for re-selection; means for receiving system information sentby a new cell. In addition, the subscriber terminal comprises means forcalculating the time it takes to receive the system information of thenew cell using the length information in a system information part sentby the new cell.

The invention also relates to a network part of a cellular networkcomprising means for sending system information of a cell. In additionthe network part comprises means for placing length informationindicating the system information length into a system information part.

The preferred embodiments of the invention are disclosed in thedependent claims.

The idea of the invention is that system information contains the systeminformation length. On the basis of the length information thesubscriber terminal can calculate how long it takes to receive thesystem information.

Several advantages are achieved with the method and system of theinvention. The system information length information included in thesystem information enables an open way to map system informationelements onto a logical control channel. The network operator can maponly the necessary information elements onto the logical controlchannel, without limiting the total number of elements.

The subscriber terminal can estimate on the basis of the received systeminformation length how long it takes to re-select a cell to said cell.Likewise the network part naturally knows how long it takes to re-selecta cell to each cell.

Estimation of the cell re-selection time enables the functions of thenetwork part and the subscriber terminal to be controlled before, duringand after cell re-selection. For example, if the estimated time isexceeded by a certain percentage cell re-selection can be interruptedand re-started from the beginning, possibly with another cell. The useror the application employed by the user can also be provided withinformation about a starting cell re-selection, during which a breakwill occur in data transmission. The invention can also be utilized inbattery or memory saving routines.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following the invention will be described in greater detail inconnection with the preferred embodiments and with reference to theaccompanying drawings, in which

FIG. 1A is a block diagram showing a cellular network,

FIG. 1B shows a circuit-switched connection,

FIG. 1C shows a packet-switched connection,

FIG. 2 shows the structure of a transceiver,

FIG. 3 illustrates the principle of cell re-selection,

FIGS. 4A and 4B form a flow chart illustrating a cell re-selectionmethod of the invention,

FIGS. 5A and 5B show examples of mapping system information into radiopackets, and

FIGS. 6A, 6B, 6C, 6D and 6E depict calculated cell re-selection timesusing different mapping parameters.

DETAILED DESCRIPTION OF THE INVENTION

With reference to FIG. 1 a typical cellular network structure of theinvention and its interfaces to a fixed telephone network and a packettransmission network are described. FIG. 1 comprises only the blocksthat are essential for describing the invention, but for those skilledin the art it is obvious that a conventional cellular network alsoincludes other functions and structures that will not be described herein greater detail. The invention is most preferably used in a GSM phase2+ packet transmission, i.e. in GPRS (General Packet Radio Service).GPRS (General Packet Radio Service) is a new GSM-based service, whereair interface capacity not used in circuit-switching is employed forpacket transmission.

A cellular network typically comprises a fixed network infrastructure,or a network part, and subscriber terminals 150, which may be fixedlymounted, vehicle mounted or hand-held portable terminals. The networkpart comprises base stations 100. Several base stations 100 are, inturn, controlled in a centralized manner by a base station controller102 communicating with them. The base station 100 comprises transceivers114, typically 1-16 transceivers 114. One transceiver 114 offers radiocapacity to one TDMA frame, i.e. typically to eight time slots.

The base station 100 comprises a control unit 118 controlling theoperation of the transceivers 114 and a multiplexer 116. The multiplexer116 arranges traffic and control channels used by multiple transceivers114 to a single data link 160.

There is a connection from the transceivers 114 of the base station 100to an antenna unit 112 implementing a bidirectional radio connection 170to a subscriber terminal 150. The structure of the frames to betransmitted on the bidirectional radio connection 170 is also accuratelydetermined and referred to as an air interface.

The subscriber terminal 150 can be, for example, a standard GSM mobilephone to which a laptop computer 152, which may be used in packettransmission for ordering and processing packets, can for instance beconnected by an additional card.

FIG. 2 illustrates in greater detail the structure of a transceiver 114.A receiver 200 comprises a filter blocking frequencies outside a desiredfrequency band. A signal is then converted to an intermediate frequencyor directly to baseband, in which form the signal is sampled andquantized in an analogue-to-digital converter 202. An equalizer 204compensates for interference caused by multipath propagation, forexample. From the equalized signal a demodulator 206 takes a bit streamthat is transmitted to a demultiplexer 208. The demultiplexer 208separates the bit stream from different time slots into its logicalchannels. A channel codec 216 decodes the bit stream of the differentlogical channels, i.e. decides whether the bit stream is signalling datatransmitted to a control unit 214, or speech transmitted 240 to a speechcodec 122 of the base station controller 102. The channel codec 216 alsoperforms error correction. The control unit 214 performs internalcontrol functions by controlling various units. A burst former 228 addsa training sequence and a tail to the data arriving from the channelcodec 216. A multiplexer 226 assigns to each burst its time slot. Amodulator 224 modulates digital signals to a radio frequency carrier.This function is of analogue nature and therefore a digital-to-analogueconverter 222 is needed to perform it. A transmitter 220 comprises afilter limiting the bandwidth. In addition, the transmitter 220 controlsthe output power of the transmission. A synthesizer 212 arranges thenecessary frequencies for different units. The synthesizer 212 includesa clock that may be controlled locally or in a centralized manner fromsomewhere else, for example, from the base station controller 102. Thesynthesizer 212 creates the necessary frequencies, for example, by avoltage-controlled oscillator.

FIG. 2 shows how the transceiver structure can be further divided intoradio frequency parts 230 and a digital signal processing processorincluding software 232. The radio frequency parts 230 comprise thereceiver 200, the transmitter 220 and the synthesizer 212. The digitalsignal processing processor including software 232 comprises theequalizer 204, the demodulator 206, the demultiplexer 208, the channelcodec 216, the control unit 214, the burst former 228, the multiplexer226 and the modulator 224. The analogue-to-digital converter 202 isneeded to convert the analogue radio signal into a digital signal, andcorrespondingly the digital-to-analogue converter 222 is needed toconvert the digital signal into an analogue signal.

The base station controller 102 comprises a group switching field 120and a control unit 124. The group switching field 120 is used forswitching speech and data and for connecting signalling circuits. Thebase station 100 and the base station controller 102 form a base stationsystem that comprises a transcoder 122. The transcoder 122 is generallylocated as close as possible to a mobile services switching centre 132,since speech can then be transferred in cellular network form betweenthe transcoder 122 and the base station controller 102, thus savingtransmission capacity.

The transcoder 122 converts different digital speech coding forms usedbetween a public switched telephone network and a cellular network tosuit one another, for example, from the 64 kbit/s fixed network form toanother cellular network form (e.g. 13 kbit/s) and vice versa. Thecontrol unit 124 performs call control, mobility management, statisticaldata collection and signalling.

The structure of the subscriber terminal 150 can be described utilizingthe description of the transceiver 114 structure in FIG. 2. Thestructural parts of the subscriber terminal 150 are functionally thesame as the ones in the transceiver 114. In addition, the subscriberterminal 150 comprises a duplex filter between the antenna 112 and thereceiver 200 and the transmitter 220, user interface parts and a speechcodec. The speech codec is connected to the channel codec 216 through abus 240.

As FIG. 1A shows the group switching field 120 can perform switching(depicted by black spots) to a public switched telephone network (PSTN)134 through the mobile services switching centre 132 and to a packettransmission network 142. A typical terminal 136 in the public switchedtelephone network 134 is an ordinary or an ISDN (Integrated ServicesDigital Network) phone.

The connection between the packet transmission network 142 and the groupswitching field 120 is established by a support node (SGSN=Serving GPRSSupport Node) 140. The aim of the support node 140 is to transferpackets between the base station system and a gateway node (GGSN=GatewayGPRS Support Node) 144, and to keep record of the location of thesubscriber terminal 150 within its area.

The gateway node 144 connects the packet transmission network 142 and apublic packet transmission network 146. An Internet protocol or an X.25protocol can be used at the interface. By encapsulating the gateway node144 hides the internal structure of the packet transmission network 142from the public packet transmission network 146, so for the publicpacket transmission network 146 the packet transmission network 142resembles a sub-network, the public packet transmission network beingable to address packets to the subscriber terminal 150 placed thereinand to receive packets therefrom.

The packet transmission network 142 is typically a private network usingan. Internet protocol carrying signalling and tunnelled user data. Thestructure of the network 142 may vary operator-specifically regardingthe architecture and protocols below the Internet protocol layer.

The public packet transmission network 146 may be, for example, a globalInternet, to which a terminal 148, for example a server computer, with aconnection thereto wants to transfer packets to the subscriber terminal150.

At the air interface 170 time slots not allocated to circuit-switchedtransmission are typically used for packet transmission. Capacity isdynamically allocated for packet transmission, so when a datatransmission request arrives any free channel can be allocated to beused in packet transmission. The arrangement is flexible,circuit-switched connections taking priority over packet data links.When necessary, circuit-switched transmission cancels outpacket-switched transmission, i.e. a time slot engaged in packettransmission is passed on to circuit-switched transmission. This ispossible, since packet transmission endures such interruptions well; thetransmission is proceeded in another time slot allocated for use. Thearrangement can also be implemented in such a manner that no definitepriority is given to circuit-switched transmission, but bothcircuit-switched and packet-switched transmission requests are served intheir order of arrival.

FIG. 1B describes how a circuit-switched data link is establishedbetween the subscriber terminal 150 and the terminal 136 of the publicswitched telephone network. A bold line shows how data is carriedthrough the system at the air interface 170, from the antenna 112 to thetransceiver 114, and from there multiplexed in the multiplexer 116 alongthe data link 160 to the group switching field 120, where a connectionis established to the output heading to the transcoder 122, and fromthere onwards through the connection performed in the mobile servicesswitching centre 132 to the terminal 136 connected to the publicswitched telephone network 134. In the base station 100, the controlunit 118 controls the multiplexer 116 in the transmission, and in thebase station controller 102 the control unit 124 controls the groupswitching field 120 to perform the correct connection.

FIG. 1C shows a packet-switched data link. A laptop computer 152 is nowconnected to the subscriber terminal 150. A bold line describes how datato be transferred is carried from a server computer 148 to the laptopcomputer 152. Data can naturally also be transferred in the oppositedirection, from the laptop computer 152 to the server computer 148. Datais carried through the system at the air interface 170, from the antenna112 to the transceiver 114 and from there multiplexed in the multiplexer116 along the data link 160 that is free of circuit-switched datatransmission to the group switching field 120, where a connection isestablished to the output heading to the support node 140. From thesupport node 140 data is applied along the packet transmission network142 through the gateway node 144 and is connected to the server computer148 connected to the public packet transmission network 146.

In FIG. 1B one time slot is used for circuit-switched transmission, butin FIG. 1C the free capacity of the circuit-switched data link 160corresponding to all available time slots of the air interface 170 canbe used. For the sake of clarity, a case where both circuit-switched andpacket-switched data are simultaneously transferred is not described inFIGS. 1A and 1B. However, this is possible and very common, since thecapacity free from circuit-switched data transmission can flexibly beused to implement packet-switched transmission or packet-switchedsignalling. Such a network can also be constructed, wherecircuit-switched data is not transferred at all, only packet data. Thenthe structure of the network can be simplified.

FIG. 3 illustrates the principle of cell re-selection. The GSM cellularnetwork spectrum is located between 890-960 MHz. The uplink directionemploys the frequency range 890-915 MHz and the downlink direction thefrequency range 935-960 MHz. In practice, it should be noted that aparticular operator is able to use only a certain part of the totalspectrum. The carrier spacing is 200 kHz. The duplex spacing of theuplink and downlink directions is 45 MHz. In a current cell 300A thesubscriber terminal 150 listens to control. channels at the frequency947.2 MHz. The neighbour base stations 300B, 300C, 300 D, 300E, 300F,300G are indicated on the control channel, for example, in the GPRS onthe PBCCH (Packet Broadcast Control Channel), as well as the frequenciesof the downlink control channels of the neighbour base stations 935.2MHz, 937.2 MHz, 939.2 MHz, 941.2 MHz, 943.2 MHz, 945.2 MHz, on which thesubscriber terminal 150 has to measure the received power of the signaland possibly some other parameters.

On the radio connection 170A the subscriber terminal 150 listens tocontrol channels of the current cell 300A. In addition, it regularlymeasures on unidirectional radio connections 170B, 170C, 170D, 170E,170F, 170G the received powers of the neighbour cells 300B-300G

In the GPRS the network part can order the subscriber terminal toperform MM (Mobility Management) measurements in ready state or inpacket idle mode. In addition to the listening radio connections170A-170G described in FIG. 3, the subscriber terminal 150 may also havea bidirectional radio connection in progress with the current cell 300A.The subscriber terminal 150 is referred to as being in connected mode orin non-connected mode, depending on whether it has a bidirectional radioconnection in progress or not. On the basis of the measurements thesubscriber terminal 150 can make a decision about cell re-selection, orthe network part can make the decision. Cell re-selection refers to asimilar process as handover in a standard GSM system.

In the example in FIG. 3, when the subscriber terminal 150 moves fromthe current cell 300A to a new cell 300C, the received power of thecontrol channels of the new cell 300C exceeds the received power of thecontrol channels of the current cell 300A. The subscriber terminal 150should in general communicate with the cell offering the best service.Cell re-selection is then performed, i.e. a new cell 300C becomes thecurrent cell. In cell reselection the subscriber terminal 150 has tolisten to the system information of the new cell 300C from the controlchannels of the new cell 300C; the system information being referred toas packet system information in the GPRS.

If the subscriber terminal has no active connection, i.e. packettransmission is not in progress, it does not have to signal anyinformation to the network part in connection with cell re-selection, ifthe routing area thereof remains unchanged. If the routing area changes,then the subscriber terminal has to signal a new cell to the networkpart. When the connection is active the subscriber terminal has tosignal a cell update to the network part.

As noted above the structure and length of the system information sentby various cells in a cellular-network using GPRS may vary considerably.Then, the subscriber terminal 150 does not know in advance how long ittakes to read the system information. In demanding packet transmissionapplications this may result in a situation where a fairly long breakmay occur in packet transmission. The user may interpret the delay aspoor quality of service.

The length of the system information sent on the PBCCH in the GPRS mayvary from 3 to over 70 different system information elements. Thiscauses uncertainty to the time it takes to re-select a cell, since it isnot known in advance how much system information the subscriber terminalhas to receive from the new cell 300C. The system information elementsare:

-   -   PSI1    -   PSI2(0-7)    -   PSI3    -   PSI3bis(0-15)    -   PSI4(0-7)    -   PSI5(0-7) p There are six different elements. The numbers 0-7,        0-15 in brackets indicate the possible number of different        instances of said elements.

Another factor affecting cell re-selection is how frequently an elementis mapped onto the PBCCH. If this frequency is low, the cellre-selection time increases.

A third affecting factor is the system information mapping scheme used.By selecting a fast mapping scheme the re-selection time decreases.There are some factors that affect the mapping speed:

-   -   The same system information element should not be mapped several        times during a system information mapping period. One mapping        period refers to the time during which one occurrence of each        system information element has been received.    -   The time between each system information element transmission        should be kept at a minimum.

The network operator may use different measures to adjust the factorsaffecting the cell re-selection time, and thus control the re-selectiontime. These different factors can be selected so as to optimize the useof the cellular network.

FIGS. 4A and 4B describe the method of cell re-selection in a cellularnetwork. The Figures form a flow chart when the bottom of FIG. 4A andthe top of FIG. 4B are placed next to each other.

The method starts from block 400.

In block 402 the subscriber terminal measures received powers ofneighbour cells in accordance with the system information received fromthe current cell.

In block 404 the subscriber terminal or the network part decides whethercell re-selection is needed based on the received powers measured by thesubscriber terminal. If cell re-selection is not performed thesubscriber terminal continues the measurements in block 402.

If cell re-selection is performed, then one of the neighbour cells isselected as the new cell in block 406. The cell offering the bestreceived power is, for example, selected as the new cell.

Thereafter in block 408 the subscriber terminal receives a part of thesystem information sent by the new cell. The system information elementsare sent on the PBCCH, and one of the elements, preferably an elementreferred to as PSI1 contains the system information length, i.e. afigure indicating the number of system information elements.

In block 410 the subscriber terminal decodes the received PSI1 element.

In block 412 the time it takes to receive the system information of thenew cell is calculated using the length information in the systeminformation part sent by the new cell. In order to perform thecalculation, information is also needed on the organization of thesystem information, for example information on the multiframe length,the number of radio blocks used for transmitting system information inone multiframe and on the repeat period of the system information part.These parameters will be explained in more detail below.

Optionally it is decided in block 414 on the basis of the calculatedtime whether re-selection of said cell is continued. If re-selection isnot desired, then the process proceeds to block 406 to select anotherneighbour cell as the new cell.

When it is desired to continue cell re-selection the process proceeds toan optional block 416 where the user is provided with information aboutcell re-selection. An example of the information provided is that packettransmission is interrupted for a calculated time. This has theadvantage that the user believes that the service obtained is of higherquality. The application performing packet transmission can also beinformed about cell re-selection.

Next in block 418 the following system information element is received,and in block 420 the received element is decoded.

In block 422 it is checked if all system information elements needed arereceived. If so, then cell re-selection is successfully performed, andthe process can return to block 402, and start measuring received powersof neighbour cells in accordance with the system information receivedfrom the new cell.

If all elements are not yet received, then the time it has actuallytaken to receive the system information of the new cell can optionallybe compared in block 424 with the time calculated in block 412.Re-selection of said new cell is interrupted if the actual time exceedsthe calculated time. A safety margin, for example 20%, can also bedetermined to the calculated time, in which case the interruption is notperformed until the actual calculated time is exceeded in reality by 20percent. When interruption occurs the process returns to block 406 whereanother neighbour cell is selected as the new cell. If no interruptionoccurs, then the process continues from block 418, where the followinginformation element is received.

In the following an example will illustrate what the system informationelement referred to as PSI1 (Packet System Information Type One) may beas described by CSN.1: < PSI1 message content > ::= < PSI1 message type: bit (6) > { L | H < Global TFI > : Global TFI IE) > } < Commonparameters : Common parameters struct > < PRACH Control Parameters :PRACH Control Parameters IE > < Control Channel Description : ControlChannel Description struct > < Global Power Control Parameters : GlobalPower Control  Parameters IE > < spare padding > ; < Common parametersstruct > ::= < BCCH_CHANGE_MARK : bit (3) > < PBCCH_CHANGE_MARK : bit(3) > < PSI_COUNT : bit (6) > < BA_GIND : bit (1) > <NETWORK_CONTROL_ORDER1 : bit (1) > < BS_CV_MAX : bit (4) > <CONTROL_ACK_TYPE : bit (1) ; { 0 | 1 < PAN_DEC : bit (3) > < PAN_INC :bit (3) > < PAN_MAX : bit (3) > } ; < Control Channel Descriptionstruct > ::= < BS_PBCCH_BLKS : bit (2) > { 0 | 1 < BS_PCC_CHANS : bit(4) > } { 0 | 1 < BS_PAG_BLKS_RES : bit (4) > } { 0 | 1 < BS_PRACH_BLKS: bit (4) > } < DRX_TIMER_MAX : bit (3) > < EXT_DYN_ALLOCATION_SUPPORTED: bit (1) > < FIXED_ALLOCATION_SUPPORTED : bit (1) > < CONTROL_CH_REL :bit (1) >

A six bit parameter referred to as PSI_COUNT (Packet System InformationCount) is a parameter according to the invention indicating how manydifferent system information elements the subscriber terminal has toreceive from said cell in order to obtain all the necessary systeminformation.

Next another example of a possible PSI1 structure is described by CSN.1:< PSI1 message content > ::= < PAGE_MODE : bit (2) > < PBCCH_CHANGE_MARK: bit (3) > < PSI_CHANGE FIELD : bit (4) > < PSI1_REPEAT_PERIOD : bit(4) > < PSI_COUNT_LR : bit (6) > { 0 | 1 < PSI_COUNT_HR : bit (4) > } <MEASUREMENT_ORDER : bit (1) > < GPRS Cell Options : GPRS Cell OptionsIE > < PRACH Control Parameters : PRACH Control Parameters IE > < PCCCHOrganization Parameters : PCCCH Organization Parameters IE > < GlobalPower Control Parameters : Global Power Control Parameters IE > <PSI_STATUS_IND : bit > < padding bits > ! < Distribution part error :bit (*) = < no string > >;

In this example the parameter PSI_COUNT consists of two parameters, aPSI_COUNT_LR (LR=Low Rate) and an optional PSI_COUNT_HR (HR=High Rate).These two parameters are added together in order to obtain thePSI_COUNT.

FIG. 5A depicts two different mapping schemes. Different parameters areselected for each scheme so that the parameter effect can be illustratedat the cell re-selection time.

In the description below a 51-multiframe is used as an example, but theprinciples hold true also for other types of multiframes, for examplefor a 52-multiframe. The 51-multiframe is shown in the horizontaldirection, frame number zero of the 51-multiframe is shown on the left,and frame number fifty of the 51-multiframe on the right. X illustratesframes in which information elements cannot be placed, since they areallocated for other purposes, like transferring timing information orfrequency correction information. X indicates one frame, and XX twoframes. Radio blocks B0, B1, B2, B3, B4, B5, B6, B7, B8 and B9 are eachfour TDMA frames in length owing to interleaving that is performed amongfour TDMA frames. Information elements can be placed in radio blocksB0-B9.

M refers to the value that is used for calculating TC. TC is used forlocating a PSI1 message. In the Figure TC is described vertically as anincreasing figure. A first 51-multiframe is at first sent using TC valuezero, then a second 51-multiframe is sent using TC value one etc. M canalso be referred to as a repeat period value, since it describes at what51-multiframe intervals the PSI1 message repeats itself. The calculationformula is:TC=(FN DIV MFL) mod M,  (1)

where DIV is integer number division,

mod is modulo,

FN is multiframe number (0-2715647)

MFL is multiframe length (51 or 52),

M varies between 1-16, assuming that it cannot be 1, if BS is 1 or 2.

In FIG. 5A BS refers to a number by which the network part informs thesubscriber terminal into how many radio blocks system information ismapped in one 51-multiframe.

In the example above the service provider has selected value four asmapping parameter M and value one as mapping parameter BS. PSI_COUNTobtains the value six. Slightly more than six 51-multiframes are usedfor receiving the described information elements PSI1, PSI2(0), PSI2(1),PSI2(3), PSI1 repeat, PSI3 and PSI4. The cell selection time can becalculated as 1441.4 milliseconds.

In the example below the service provider has selected value five asmapping parameter M and value four as mapping parameter BS. PSI_COUNTobtains the value twenty-one. The PSI1 message is thus not repeated asfrequently as in the above example. A fourfould amount of the transfercapacity of one 51-multiframe is used compared with the example above.Therefore slightly less than six 51-multiframes are used for receivingthe described information elements PSI1, PSI2(0), PSI2(1), PSI1 repeat,PSI2(2), PSI2(3), PSI2(4), PSI2(5), PSI2(6), PSI2(7), PSI3, PSI3(0),PSI3B(1), PSI3B(2), PSI3B(3), PSI3B(4), PSI3B(5), PSI3B(6), PSI3B(7),PSI4(0), PSI4(1), PSI4(2), PSI1 repeat, PSI4(3), PSI4(4), and PSI1repeat. The cell selection time can be calculated as 1316.7milliseconds. The letter B in the information elements refers to bis.

Even though the subscriber terminal has to receive much more systeminformation in the example below than in the example above, thereselection time is shorter in the example below. This is caused by theselected mapping parameter values.

In the following an example is presented of the rules the subscriberterminal should use in order to know how the network part sends thepacket system information. The messages are sent in determinedmultiframe radio blocks. The message occurrences are determined usingthe previously presented formula 1. The basic rules are:

1. PSI1 is sent in B0 using TC value zero.

2. If BS>1, then PSI1 appears only twice in the multiframe using TCvalue 0. The second PSI1 occurrence using TC value 0 is in the lastavailable radio block of the multiframe.

In addition to the basic rules various rules can be determined on howthe rest of the packet system information is mapped, for example:

3. The rest of the packet system information is mapped to the availableradio blocks using the rule: All existing instances of PSIx and PSIxbis(where x=2, 3, 4, 5) are placed into the radio blocks which are to beused in ascending order.

4. The rest of the unfilled available radio blocks are filled inaccordance with rule 3 starting from PSI1.

FIG. 5B shows an example of how values M eight, BS four, and PSI_COUNTtwenty-eight can be used to place the system information elements into a51-multiframe using the four rules above. The available radio blocks areB0, B2, B5 and B7. According to rule 1, PSI1 is placed in B0 using TCvalue zero. According to rule 2, PSI1 repeat is placed into the lastavailable radio block, or B7, using TC value zero. The rest of theinformation elements are placed in accordance with rule 3 in ascendingorder starting from TC value zero from B2 and ending at TC value 7 inB0. In accordance with rule 4 system information elements starting fromPSI1, i.e. elements PSI1, PSI2(0) and PSI2(1) are placed in ascendingorder into the rest of the radio blocks, or B2, B5 and B7.

An example is shown below of an algorithm by which the network part orthe subscriber terminal can calculate the cell re-selection time inideal circumstances. Ideal means that all system information elementsare correctly received the first time, and that no mistakes occur duringreception. The algorithm is written with a notation understood byMatlab™ program.

-   -   Input parameter from the network system information elements        used for calculating the ideal cell reselection time:    -   MFL=MultiFrame Length    -   BS=BS_PBCCH_BLKS;    -   N=PSI_COUNT    -   M=PSI1_REPEAT_PERIOD    -   Assumption:

PSI1_REPEAT_PERIOD cannot equal 1 if BS_PBCCH_BLKS equal 1 or 2.  1. if(BS == 1 | BS == 2 )  2. I = BS*(M − 1);  3. end  4. if( BS == 3 )  5. I= BS*(M − 1) + 1;  6. end  7. if( BS == 4 )  8. I = BS*(M − 1) + 2;  9.end 10. if( BS > 1) 11. N1 = N + 1; 12. else 13. N1 = N; 14. end 15. M1= 0; 16. Q = 1; 17. s=0; 18. while( Q == 1 ) 19. if( N1 > BS ) 20. M1 =M1 + 1; 21. N1 = N1 − BS; 22. if( BS == 3 | BS == 4 ) 23. if( rem(M1,M)== 0 ) 24. N1 = N1 + 2; 25. s = s + 1; 26. end 27. end 28. else 29. Q =0; 30. if( M1 == 0 & BS > 1 ) 31. N1 = N1 − 1; 32. end 33. end 34. end35. if( s > 0 ) 36. if( BS == 3 | BS == 4 ) 37. if( rem(M1,M) == 0 ) 38.N1 = N1 − 1; 39. end 40. end 41. M1 = M1 − s; 42. end 43. if( N > ( I +1 ) ) 44. if( BS > 1 ) 45. R = N − ( M*BS ); 46. else 47. R = N − M − 1;48. if( R < 0 ) 49. R = 0; 50. end 51. end 52. m = fix(R / I) + 1; 53.M1 = M1 + m; 54. end 55. if( N1 == 1 ) 56. if( MFL == 51 ) 57. x = 6;58. else 59. x = 4; 60. end 61. end 62. if( BS == 2 ) 63. if( N1 == 2 )64. x = 30; 65. end 66. end 67. if( BS == 3 | BS == 4 ) 68. if( N1 == 2) 69. if( MFL == 51 ) 70. x = 16; 71. else 72. x = 17; 73. end 74. else75. if( N1 == 3 ) 76. x = 30; 77. end 78. end 79. end 80. if( N1 == 4 )81. if( MFL == 51 ) 82. x = 40; 83. else 84. x = 43; 85. end 86. end 87.T1 = MFL_TIME * M1; 88. T2 = TDMA_TIME * x; 89. T = T1 + T2;

-   -   Fixed parameter:    -   TDMA_TIME=1 burst time;    -   Variable:    -   MFL_TIME=MFL*TDMA_TIME;    -   Variable explanation:    -   M: Parameter available from the network system information. Used        to calculate the TC-value.    -   Note: This value is the same as the mentioned        PSI1_REPEAT_PERIOD.    -   PSI_COUNT: Parameter available from the network system        information.    -   And:    -   rem(x,y)    -   equals modulus division (MOD), and:    -   fix(x,y)    -   equals integer division (DIV), and    -   T    -   equals the actual ideal cell re-selection time.

FIGS. 6A, 6B, 6C, 6D, 6E illustrate the cell re-selection times usingdifferent mapping parameters calculated by the above algorithm. ThePSI_COUNT value is described on the X-axis and the cell re-selectiontime in milliseconds on the y-axis. In all Figures the PSI_COUNT obtainsvalues between 1-28 and MFL is 51.

In FIG. 6A M is 4. Basically, four values are calculated for eachPSI_COUNT value, the BS obtaining the values 1, 2, 3 and 4. The dotsform four curves, the lowest one corresponding to BS value 4, the secondlowest to BS value 3, the third lowest to BS value 2 and the highest toBS value 1. The more frequently information elements are sent, thefaster the cell re-selection can be performed.

FIGS. 6B, 6C, 6D and 6E describe the effect of changing the M value. Thelower curve of dots corresponds to M value 16 and the higher curve to Mvalue 4. In FIG. 6B, BS obtains the value 1, in FIG. 6C the value 2, inFIG. 6D the value 3 and FIG. 6D the value 4. The more radio blocks eachmultiframe uses, the less the M value affects the cell re-selectiontime.

The example above was based on the presented basic rules. It is obviousthat one skilled in the art may apply other rules too, in order toimplement the invention. In the following an example of different rulesis presented:

1. PSI shall be sent in block B0 when TC=0.

2. If the value of the parameter BS_PBCCH_BLKS is greater than 1, thePSI1 shall also be sent in block B6 (for 52-multiframe) or B5 (for51-multiframe) when TC=0.

3. The PSI messages in the group sent with a high repetition rate shallbe sent in a sequence determined by the network and starting at TC=0,using the PBCCH blocks within each multiframe which are not occupiedaccording to rule 1 or 2. The sequence of these PSI messages shall berepeated starting at each occurrence of TC=0.

4. The PSI messages in the group sent with a low repetition rate shallbe sent in a sequence determined by the network and continuouslyrepeated, using the PBCCH blocks within each multiframe which are notoccupied according to rules 1 to 3.

If there are multiple instances of a particular PSI message type, theyshall all be sent within the same PSI message group according to eitherrule 3 or 4 above. They shall be sent in a single sequence in theascending order of the message instance number of that PSI message type.

The same PSI message shall not occur twice within the lists defined bythe PSI_COUNT_LR and PSI_COUNT_HR.

A full set of Packet System Information messages contains one consistentset of the messages included in PSI_COUNT_LR and one consistent set ofmessages included in PSI_COUNT_HR plus the PSI1 message.

The invention is preferably implemented by software. The invention thenrequires relatively simple software changes within a strictly restrictedarea in the network part and the subscriber terminal. The subscriberterminal comprises means for measuring received powers of neighbourcells in accordance with the system information received from thecurrent cell, means for discovering the need for cell re-selection,means for receiving system information sent by the new cell, and meansfor calculating the time it takes to receive the system information ofthe new cell based on the length information in the system informationpart sent by the new cell. The network part comprises means for sendingcell system information, and means for placing the length informationindicating the system information length into a part of the systeminformation. The matter disclosed in the dependent method claims cancorrespondingly be implemented by the means performing the operation.The means are preferably implemented as software, for example, as asoftware to be carried out in a processor or as an ASIC (ApplicationSpecific Integrated Circuit). In the subscriber terminal the means areimplemented by a processor including software 232, for example. In thenetwork part the means can be divided differently depending on theresponsibilities between the control unit 118 of the base station 100,the control unit of the base station controller 102 and possibly alsothe support node 140.

Even though the invention has been described above with reference to theexample of the accompanying drawings, it is obvious that the inventionis not restricted thereto but can be modified in various ways within thescope of the inventive idea disclosed in the attached claims.

1. A method for performing cell re-selection in a cellular network,comprising: a subscriber terminal measuring received powers of neighbourcells in accordance with system information received from a currentcell; selecting one of the neighbour cells as a new cell; the subscriberterminal receiving a part of the system information sent by the newcell; calculating the time used for receiving the system information ofthe new cell by employing the length information in the systeminformation part sent by the new cell; and deciding whether to continuethe re-selection of said new cell on the basis of the calculated time.2. The method of claim 1, wherein calculating the time comprises usinginformation on multiframe length.
 3. The method of claim 1, whereincalculating the time comprises using information on the number of radioblocks used for sending system information in one multiframe.
 4. Themethod of claim 1, wherein calculating the time comprises usinginformation on a repeat period of the system information part.
 5. Themethod of claim 1, further comprising providing the user withinformation associated with cell re-selection.
 6. A method forperforming cell re-selection in a cellular network, comprising: asubscriber terminal measuring received powers of neighbour cells inaccordance with system information received from a current cell;selecting one of the neighbour cells as a new cell; the subscriberterminal receiving a part of the system information sent by the newcell; calculating the time used for receiving the system information ofthe new cell by employing the length information in the systeminformation part sent by the new cell; comparing the actual time spentfor receiving the system in-formation of the new cell with thecalculated time; and interrupting the re-selection of said new cell, ifthe actual time spent exceeds the calculated time.
 7. The method ofclaim 6, wherein the cellular network utilizes GPRS and the methodfurther comprises placing the system information on a PBCCH.
 8. Themethod of claim 7, wherein the system information is formed of systeminformation elements comprising an element referred to as PSI1containing the system information length as a figure indicating thenumber of system information elements.
 9. The method of claim 7, whereinthe PBCCH is placed in at least one radio block, four TDMA frames long,in each multiframe.
 10. A subscriber terminal comprising: a radioconnection to a current cell base station of a cellular network; and acontrol unit for controlling functions of the subscriber terminal, thecontrol unit being configured to measure received powers of neighbourcells in accordance with system information received from a currentcell, to discover the need for cell re-selection, to receive systeminformation sent by a new cell, to calculate the time it takes toreceive the system information of the new cell using the lengthinformation in a system information part sent by the new cell, and toutilize the calculated time in the cell reselection.
 11. The subscriberterminal of claim 10, wherein the control unit is further configured todecide whether to continue the re-selection of said new cell on thebasis of the calculated time.
 12. The subscriber terminal of claim 10,wherein the control unit is further configured to compare the timeactually spent for receiving the system information of the new cell withthe calculated time.
 13. The system of claim 12, wherein the controlunit is further configured to interrupt the re-selection of said newcell if the actual time spent exceeds the calculated time.
 14. Acomputer program product comprising a computer usable medium including acomputer program of instructions, wherein when executed on a computerthe computer program causes the computer to perform cell re-selection ina cellular network by: measuring received powers of neighbour cells inaccordance with system information received from a current cell;selecting one of the neighbour cells as a new cell; receiving a part ofthe system information sent by the new cell; calculating the time usedfor receiving the system information of the new cell by employing thelength information in the system information part sent by the new cell;and utilizing the calculated time in the cell re-selection.
 15. Thecomputer program product of claim 14, comprising deciding on the basisof the calculated time whether to continue the re-selection of said newcell.
 16. The computer program product of claim 14, comprising comparingthe actual time spent for receiving the system information of the newcell with the calculated time.
 17. The computer program product of claim16, comprising interrupting the reselection of said new cell if theactual time spent exceeds the calculated time.
 18. A network part of acellular network comprising: means for sending system information of acell; and means for placing information indicating the systeminformation length into a part of the system information.
 19. Thenetwork part as claimed in claim 18, further comprising means forcalculating the time it takes for a subscriber terminal to receivesystem information using the length information.